Conventionally, in order to produce a quaternary alkyl ammonium salt, for example, to produce quaternary alkyl ammonium tetrafluoroborate, quaternary alkyl ammonium chloride, bromide or hydroxide is allowed to react with fluoroboric acid. Such methods have been proposed in U.S. Pat. No. 3,965,178 (1976), and published Japanese Patent Applications, Publication No. S63(1988)-30454, 2000-226361, and 2001-247522.
A published Japanese Patent Application, Publication No. H5(1993)-286981 proposes a method involving the use of anhydrous hydrofluoric acid. An published Japanese Patent Application, Publication No. H11(1999)-310555 proposes yet another method in which a hydrogenfluoride salt of quaternary alkyl ammonium fluoride and boron trifluoride or its complex are allowed to react. A published Japanese Patent Application, Publication No. H11(1999)-27179 proposes yet another method in which tertiary amine is allowed to react with alkyl halide, which is immediately followed by reaction with fluoroboric acid. A published Japanese Patent Application, Publication No. H11(1999)-315055 proposes yet another method in which quaternary alkyl ammonium bicarbonate and fluoroboric acid are allowed to react.
A quaternay alkyl ammonium salt (e.g., quaternary alkyl ammonium tetrafluoroborate) obtained by one of the conventional methods described above contains a trace amount of chlorine or bromine derived from a starting material, or of fluoroboric acid which is introduced during the synthetic process. If such a salt is not properly treated so as to be deprived of those contaminants, the salt could hardly be used with profit as an electrolyte in an organic electrolytic solution of a dielectric capacitor or battery. The method for treating such a salt to improve its purity includes purification based on recrystallization which, however, can hardly achieve the complete elimination of contaminants.
A published Japanese Patent Application, Publication No. 2001-348388 discloses a method in which quaternary alkyl ammonium halide is allowed to react with fluoroboric acid in alcohol, and the reaction product is dried. This method enables the elimination of chlorine or bromine contaminant derived from the starting material, and thus the obtainment of a quaternary alkyl ammonium salt in which contaminants derived from the starting materials are significantly inhibited.
A quaternary alkyl ammonium salt obtained by this method, however, tends to conglomerate as do quaternary alkyl ammonium salts obtained by the conventional methods described above. Because of this tendency, the salt in question poses following problems.
Quaternary alkyl ammonium salts are generally highly hygroscopic. Because of this property, quaternary alkyl ammonium salts obtained by the conventional methods tend to conglomerate, which reduces the drying efficiency of the salts. If such a quaternary alkyl ammonium salt is used as an electrolyte in an electrolytic solution of an electric circuit element, it will react with water to produce an acid in the electrolytic solution which will result in the augmented internal resistance if the circuit element is a battery, or corrode the metal parts, if the circuit element is a dielectric capacitor.
An object of the present invention is to solve the problems inherent to the conventional methods. To put it more specifically, the object of the present invention is to provide a method for producing highly pure quaternary alkyl ammonium salts which can be profitably used in the production of dielectric capacitors.